The present invention relates to wood structures and in particular to a structure formed with composite wood members including aligned pairs of elongated pieces each having a trapezoidal cross-sectional geometry, a narrow face and a wide face. The narrow faces of the pieces are bonded together in mutually opposing or abutting relationship between the wide faces. The composite wood member is more efficient to produce and more efficient structurally than comparably sized boards having rectangular cross-sectional configurations and may be used in place of such boards in many applications. The present invention also relates to a method of producing composite wood members from logs with minimal waste and maximum yield.
In the lumber field, wood structural members or boards having rectangular cross-sectional configurations and comprising a single piece cut from a log are well known. Such members or boards are generally designated by their nominal cross-sectional dimensions rounded off to the next largest inch although the actual finished dimensions after milling and drying are less. For example, boards commonly designated as 2.times.4's have nominal cross-sectional dimensions of two inches and four inches and actual cross-sectional dimensions of approximately one and one-half inches and three and one-half inches. Such 2.times.4 boards of appropriate length may be placed vertically at a predetermined spacing as studs within wall structures with other materials attached to their one and one-half inch wide opposite faces to form walls. Also, boards with nominal cross-sectional dimensions of, for example, two inches and ten inches (2.times.10's) may be placed horizontally in various structures to support finished floor surfaces on their upper one and one-half inch wide faces. As a further example of common uses for such boards, rafters comprising boards with nominal cross-sectional dimensions of two inches and six inches (2.times.6's) may be tilted to an appropriate angle to support a roof structure. Thus, for a typical wood-frame structure, several sizes of boards may be utilized for different portions thereof.
Such wood members having rectangular cross sections and comprising a single piece cut from a log, however, have several significant drawbacks. A prior art method of cutting the log to produce such boards involves feeding it lengthwise through a saw a number of times with the cuts being substantially parallel or perpendicular with respect to each other. Using this method, solid wood slabs or edgings comprising areas adjacent the log's cylindrical or frusto-conical outer surface remain after sawing out the boards and are generally suitable only for processing into fuel, particle board, wood pulp and the like, all of which products are less valuable on a per volume of wood basis than boards usable as wood structural members. Such slabs or edgings, together with those portions of the log consumed in the saw kerfs, may account for as much as forty to fifty percent of the wood volume in a particular log. In relatively small-diameter logs, such portions may account for even higher percentages of the total wood volume thereof. Therefore, this represents a relatively wasteful and inefficient method of producing wood members.
Another problem with producing such boards from logs is that maximum utilization of the total wood volume thereof often requires producing boards with different cross-sectional dimensions. For example, a log of given diameter might yield only a few 2.times.8's, along with 2.times.6's and 2.times.4's. Also, correspondingly larger logs are required for the production of the relatively larger boards. Therefore, boards having larger cross-sectional dimensions command higher prices per board foot than boards with smaller cross-sectional dimensions.
Furthermore, the available trees for milling into rectangular boards are generally expected to have smaller average diameters due to the fact that smaller diameter trees generally reappear in wooded areas after harvesting and reforestation. Therefore, maximizing the percentage yield of usable lumber from remaining forests is particularly important.
In addition to being relatively inefficient to produce, boards with rectangular cross-sections are structurally relatively inefficient. In use as structural framing members, boards are selected and placed as required to resist loads acting against the structure. Thus, 2.times.4's used as wall studs are sized and spaced as required to support overlying portions of the structure and to maintain the wall in a substantially straight, upright position by resisting lateral forces and any tendency of the wall to buckle under compressive loads placed thereon. Roof rafters and floor joists, on the other hand, are generally placed horizontally or at an angle from the vertical and are primarily subjected to bending loads acting downwardly thereon. To obtain maximum utilization of the strength in such rafter and joist members, faces thereof with longer cross-sectional dimensions are aligned vertically. For example, a 2.times.10 used as a floor joist would be placed with its nominal two inch wide faces horizontal and its nominal ten inch wide faces vertical.
When subjected to a bending load from above, a horizontally aligned board is subjected to compression forces in the upper half of its section modulus and tensile forces in the lower half of its section modulus. At approximately the center of a section modulus of a given structural member subjected to such a bending load, the tensile and compressive forces substantially cancel each other out and equal zero. However, the section modulus center is subjected to a longitudinal shear force caused by the opposite-acting compressive and tensile forces in its upper and lower halves respectively. Boards with rectangular cross-sections are structurally relatively inefficient because they have substantially the same transverse, cross-sectional width between their opposite upper and lower faces. Thus, while the upper and lower faces of such a member are respectively subjected to the maximum compressive and tensile forces, portions therebetween having the same width as the upper and lower faces are oversized for the lesser forces to which they are subjected. Therefore, efficient structural members designed for bending loads, unlike conventional boards, should have significantly less transverse cross-sectional widths between their opposite faces at which the maximum tensile and compressive forces occur.
Yet another disadvantage of conventional boards having rectangular cross sections is their susceptibility to cupping and warping due to the orientation of their faces with respect to the grain patterns of the logs of which they are comprised. The cross-sectional grain patterns of logs comprise pluralities of concentric annular rings. The orientation of the faces of a wooden member with respect to such annular rings is significant because wood tends to shrink approximately twice as much in a direction tangential to the annular rings, as noted in the "Wood Handbook" (Agricultural Handbook No. 72) published by Forest Products Laboratory of the U.S. Department of Agriculture (August, 1974). Wood member faces oriented tangentially to the annular rings are referred to as comprising flat grain, and wood member faces radially or perpendicularly oriented with respect to the annular rings are referred to as comprising vertical grain. Cupping and warping are caused by different amounts of shrinkage with respect to different portions of a wood member. Because even kiln drying removes only a portion of the moisture contained in green wood, the shrinking process can continue for a considerable period of time, even after incorporation of a wood member into a structure. This further shrinking can result in damage to the structure, for example, by causing finished walls to crack and doors and windows to bind.
The process by which conventional boards are sawn from logs generally results in at least several of their respective faces comprising flat grain wood and therefore being subject to relatively large amounts of shrinkage. Such boards are therefore particularly susceptible to cupping and warping for that reason and also because of the likelihood of different amounts of shrinkage across respective opposite faces.
The composite member of the present invention, on the other hand, is considerably less susceptible to warping and cupping than conventional boards having rectangular cross sections because of the orientation of the faces of its pieces with respect to the grain pattern of the wood. The radial faces of the pieces comprise vertical grain and are thereby subjected to relatively small amounts of shrinkage. Although the narrow and wide faces comprise flat grain and are therefore subject to larger amounts of shrinkage, because they are positioned opposite with respect to each other, such shrinkage amounts will be proportionately about the same and tend to cancel each other out. Also, the vertical grain radial faces of many such pieces will be wider than both the narrow and wide flat grain opposite faces thereof and the warping effect of the flat grain shrinkage will thus be minimized with respect to such pieces.
The composite member of the present invention also tends to resist cupping and warping because the arcuate cross-sectional grain patterns of the pieces are concave with respect to their narrow faces. Thus, when the pieces are bonded together with their arcuate grain patterns oriented opposite with respect to each other, the cupping and warping tendencies of the pieces will tend to cancel each other out.
Conventional boards often include knots, disease damage and sappy portions which are more likely to comprise a significant portion of a one-piece board than of a composite member wherein the effects of such flaws in the individual pieces tend to be minimized with respect to the composite member as a whole.